feat[vLLM × v5]: Add audio support for the Transformers backend

[harshaljanjani]

What does this PR do?

→ This PR adds support for v5 Transformers audio encoder models in the vLLM Transformers backend. These changes are deliberate and are blocked by this Transformers PR which adds prerequisite compatibility to the supported models for vLLM. Once that PR is merged, this PR will be marked ready for review!
→ Outlining the design choices of one PR without context from the other didn't make much sense to me, so I wrote a doc that outlines both sets of changes together and explains their deliberate nature, amongst other valuable things!
→ The v5 tracker doesn’t mention the audio backend, but it is certainly a significant gap that needs to be addressed. After this is merged, I'll open an issue tracker for the Transformers audio backend work in vLLM so the efforts can stay organized.

Please refer to the document for the reasoning behind these changes in context with the Transformers PR!
Document: v5 x vLLM Audio Backend Support Document

Performance Metrics (Env mentioned in the document)

Reference Audio Transcript:
“MISTER QUILTER IS THE APOSTLE OF THE MIDDLE CLASSES AND WE ARE GLAD TO WELCOME HIS GOSPEL”

Model	Output Text	Latency (E2E)	Throughput	Tokens
GLM-ASR-Nano-2512	"Mister Quilter is the apostle of the middle classes, and we are glad to welcome his gospel."	856.3 ms	26.9 tok/s	23
Audio-Flamingo-3-HF	"The content of the input audio is 'mister quilter is the apostle of the middle classes and we are glad to welcome his gospel'."	1779.6 ms	16.9 tok/s	30
VibeVoice-ASR-HF	[{"Start":0,"End":5.0,"Speaker":0,"Content":"Mr. Quilter is the apostle of the middle classes, and we are glad to welcome his gospel."}]	2577.9 ms	17.1 tok/s	44
Granite-Speech-3.3-2B	"Mister Quilterter is the apostle of the middle classes, and we are glad to welcome his gospel. In written format: Mister Quilterter is the apostle of the middle classes, and we are glad to welcome his gospel."	3024.9 ms	19.5 tok/s	59

Related Issues:

→ Current v5 tracker: #38379
→ #38902
→ Solved out of the box with this PR: #32823
→ Documented vLLM engine issue mentioned in the document: #17676

@vasqu (Transformers)
@DarkLight1337 @hmellor (vLLM)

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[gemini-code-assist]

Code Review

This pull request adds support for audio models to the Transformers modeling backend by unwrapping nested CausalLM structures, extending multimodal processing metadata for audio, and refactoring embedding logic to handle audio features. It also includes a comprehensive test suite for audio model processing. A critical issue was identified in the extraction of audio embeddings, where the current implementation incorrectly selects pooled outputs instead of the full feature sequence, potentially causing a mismatch with prompt placeholders.

[hmellor]

Thank you for this PR!

I'm aware that @eustlb is actually doing some refactoring on the Transformers side to make audio models look more like other multimodal models (which may render the changes in causal.py unnecessary.

We should wait for this standardisation to be completed and then we can update the PR on the vLLM side to hook into this more standardised interface.

[harshaljanjani]

I'm aware that @eustlb is actually doing some refactoring on the Transformers side to make audio models look more like other multimodal models (which may render the changes in causal.py unnecessary.

I would love to provide some extra bandwidth in that regard as well @eustlb!

We should wait for this standardisation to be completed and then we can update the PR on the vLLM side to hook into this more standardised interface.

Sure, will be on the lookout for pings and updates.

[RocketRider]

Tf5 support is now merged

[eustlb]

Thanks @harshaljanjani for working on this!
Opened #45534 to fix the VLM/ ALM discrepancy regarding base model class that should reduce changes required here

[harshaljanjani]

Thanks @harshaljanjani for working on this! Opened #45534 to fix the VLM/ ALM discrepancy regarding base model class that should reduce changes required here

Awesome stuff @eustlb, thanks for letting me know! Will let the review rounds play out for the linked PR and start work here once it's merged to avoid a dupl of efforts. Also if I recall correctly, an issue was brought to light a couple of months back in this PR with traces; I'd love to know if there has been any standardization in that regard since we postponed the hotfix at the time :)
Would love to coordinate efforts, in any case happy to provide some extra bandwidth where needed!

Edit: Marking this as ready for review since the Transformers PR has now been merged. Looking forward to the review rounds once ALM standardization is complete!

[claude]

Claude Code Review

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[harshaljanjani]

Good day @hmellor @eustlb; I refactored this PR to match the ALM standard set by #45534 + verified all the tests adjacent to the change and re-ran the user-facing benchmark on it. I should note that while verifying after removing all changes in CausalMixin, I found that AudioFlamingo3 and VibeVoice were producing garbage output: so the root cause turned out to be that AudioFlamingo3Config like configs explicitly set tie_word_embeddings=True and their text config sets it to False, so the OR in _get_tie_word_embeddings() always resolved to True and vLLM tied lm_head to the embeddings instead of loading its own weights. I made a change to fix that nit.

Before (broken):

Model	E2E (ms)	Tok/s	Tokens	Output preview
granite_speech	2959	19.9	59	Mister Quilterter is the apostle of the
audioflamingo3	11523	17.4	200	serving characteristic distant distant
vibevoice_asr	11506	17.4	200	何的ο星几ksksいてв藏某恢复 localiosfefefefefefefe
glmasr	818	28.1	23	Mister Quilter is the apostle of the mid

After (fixed):

Model	E2E (ms)	Tok/s	Tokens	Output preview
granite_speech	2912	20.3	59	Mister Quilterter is the apostle of the
audioflamingo3	1780	16.9	30	The content of the input audio is 'mister
vibevoice_asr	2576	17.1	44	[{"Start":0,"End":5.0,"Speaker":0,"Content"
glmasr	822	28.0	23	Mister Quilter is the apostle of the middle

I'm sharing the broken logs for verification against repros. Everything works the way it did before the standardization, except it's cleaner now and the changes in CausalMixin are gone. Will await reviews, thanks :)

Test commands:

pytest tests/models/multimodal/processing/test_transformers_audio.py
pytest tests/models/multimodal/processing/test_transformers.py
python benchmark_audio.py  # https://gist.github.com/harshaljanjani/d9f619683a1dfb0f41c14b4455bad514

[hmellor]

Some initial comments

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[harshaljanjani]

@hmellor Investigated the logs and re-verified there are no breakages against Transformers main. The failure was an AssertionError in the output but without the expected token IDs from the CI GPU run I can't fix it exactly, so I've added a temporary print for the token IDs for the next run you trigger, and I've fixed the VibeVoice failure. The unrelated Ultravox error will probably still fail though.

[hmellor]

I've added a temporary print for the token IDs for the next run you trigger

The output produced by HF should be the reference, not whatever the Transformers backend outputs today

I've fixed the VibeVoice failure

Looks like it's just skipped rather than fixed, can we not fix it?

[harshaljanjani]

@hmellor Sorry I should've been a bit clearer on the thought process there!
→ VibeVoice: The real fix is upstream in Transformers since its processor requires audio as a positional argument, unlike the other audio processors (AF3, GLM-ASR) that default it to None, so it can't process the text-only inputs the correctness test generates. I'll raise a PR for it upstream shortly and keep the skip as the interim until it lands, please do let me know if there's something else you'd expect that's more idiomatic, happy to change direction :)
→ Output matching: My reasoning was that since it's hardware drift I wanted to match it to the CI GPU outputs, but I think I found precedent that matches what you said (the output produced by HF should be the reference, not whatever the Transformers backend outputs today), happy to remove the fixture and replace it with this pattern instead, or is there something I'm missing?

[hmellor]

happy to remove the fixture and replace it with this pattern instead

Yeah that sounds good. This way:

• we don't need to hard code the answers
• we also get a little bit of validation for the Transformers side

[hmellor]

so it can't process the text-only inputs the correctness test generates

So it does work when provided with text and audio?

[harshaljanjani]

Yeah that sounds good. This way:

Great! I'll swap the fixture for the pattern.

So it does work when provided with text and audio?

So yeah with audio=None partly, single audio yes, multiple audios no. Since my last message I verified that if you change VibeVoice to be audio=None on the Transformers side, you have text and text + 1 audio fixed, but text + >1 audios breaks with ValueError: Got 1 text but N audios; they must match 1:1, given VibeVoice on main enforces 1:1 text-audio. My thesis is that since it diverges from the precedent set by AF3/GLM-ASR anyway and the the model itself already handles multiple audios and it's just a processor diff, my PR can standardize it like the other processors and all three cases should be unblocked here allowing us to omit the skip, but I'm still looking into it.

[harshaljanjani]

I've refactored the tests and raised the upstream PR after which the VibeVoice skip can be safely removed. I found that my current backend doesn't account for models that ship with a separate PEFT adapter, which caused a divergence in the outputs of granite-speech-3.3-2b compared to HF. I'll see what I can do to support for it as well.

[harshaljanjani]

Addressed this with a7da7, should be ready for your perusal now!

[harshaljanjani]

@hmellor Good day, removed the skip and bumped up the VibeVoice minimum version with dbdd6 since the main CI (excluding the PR-specific tests) was green except engine startup failures unrelated to this PR.

[hmellor]

Please revert that change:

• test_processing_correctness does not read min_transformers_version
• setting min_transformers_version="5.15.0.dev0" will disable the tests in CI that do

[harshaljanjani]

@hmellor Sorry, made an edit in a rush there! Coverage point makes sense and I've reverted it with f0c2a and slightly reworded the TODO for clarity. Hopefully it's alright, happy to make edits if not :)